1. Field of the Invention
The present invention relates to a microstrip coupler and a method for fabricating the same, in which a difference of an odd mode phase velocity and an even mode phase velocity of the microstrip coupler is reduced for improving a directivity, a ratio of a coupling to an isolation, of the coupler.
2. Background of the Related Art
Of the transmission lines used widely in extreme high frequency wave and microwave circuits, there is a coupled line, in which energy is exchanged through coupling between the transmission lines to transmit or cut off an energy to a particular terminal. This coupled line has two forms of mode characteristics, i.e., either in an even-mode or in an odd-mode, as the characteristics of the coupled line structure are, in which field distributions and phase velocities differ.
FIG. 1 illustrates a related art microstrip directional coupler schematically. When a power P.sub.1 is supplied to an input, one side port of the microstrip line 11, if it is assumed that a power at a through port is P.sub.2, a power at a coupled port of a coupled adjacent microstrip line 12 is P.sub.3, and a power at an isolated port is P.sub.4, a coupling C, an isolation I and a directivity D, major three parameters that form a directional coupler, can be expressed as equation (1), below. EQU C=10 log(P.sub.1 /P.sub.3)=-20 log .beta.[dB] EQU D=10 log(P.sub.3 /P.sub.4)=20 log(.beta..vertline.S.sub.14.vertline.)[dB] EQU I=10 log(P.sub.1 /P.sub.4)=-20 log(.vertline.S.sub.14.vertline.)[dB]
Where .beta. denotes a scattering coefficient .vertline.S.sub.13.vertline..
The directivity D is inversely proportional to an index D' that represents an undesired coupling, that can be expressed as equation (2). EQU D'=[.pi..DELTA.(1-.vertline..xi..vertline..sup.2)/ (4.vertline..xi..vertline.)].sup.2
where, .DELTA.=(.beta.e-.beta.o)/.beta.o EQU .xi.=.rho..sub.e /(1+.rho..sup.2.sub.e)-.rho..sub.o /(1+.rho..sup.2.sub.o) EQU .rho..sub.e =(Zoe-Zo)/(Zoe+Zo) EQU .rho..sub.o =(Zoo-Zo)/(Zoo+Zo)
.beta.e and .beta.o represent an even mode phase velocity and an odd mode phase velocity respectively, and Zoe and Zoo represent characteristic impedances in the even mode and in the odd mode, respectively. Zo represents a characteristic impedance of a transmission line, set in general at 50 .OMEGA. in a microwave measuring environment. Since a directivity of a directional coupler represents how well a forward wave component and a reverse wave component are separated, an object to which the directional coupler is applied requires a high directivity. If the directivity is poor, accuracy of a reflect meter deteriorated, to cause a change a power level P.sub.3 of the coupled port coming from the coupler even if there is a slight mismatch to the power P.sub.2 transmitted to the through line. Due to this, there have been various efforts for improving the directivity, which are directed to a method of leading the .DELTA. to zero in the equation 2, i.e., minimizing a difference between an odd mode phase velocity and an even-mode phase velocity coming from the microstrip structure which is formed in a structure having a dielectric inhomogeneity.
Referring to FIG. 2, the example of a related art microstrip coupler is provided with a substrate 10 with a height h and dielectric constant .di-elect cons.r formed on a grounded member 14, one pair of coupled microstrip lines 11 and 12 each with a width `W` and spaced `S` to each other, and an electric shielding metal 13 spaced `d` from the substrate 10 above the microstrip lines 11 and 12. The electric shield film 13 is placed in an air medium. The related art microstrip coupler having the electric shielding metal 13 redistributes a substantial amount of field in the air medium positioned above the coupled microstrip lines by means of the electric shielding metal. Therefore, since the even-mode phase velocity and the odd-mode phase velocity become the same in a condition of d=h theoretically, the above equation (2) .DELTA. [=(.beta.e-.beta.o)/.beta.o] becomes zero, to improve a directivity.
FIG. 3 illustrates a section of a related art microstrip coupler in which a directivity is improved by using lumped capacitors, schematically.
Referring to FIG. 3, another related art example is a structure in which a lumped capacitors 15 and 16 is inserted in both ends of a section of the coupled microstrip lines 11 and 12 disposed on the substrate 10. A difference between this related art example and the previous related art example is provision of the lumped capacitors 15 and 16 on both ends of the microstrip lines 11 and 12 instead of the electric shielding metal 13 of FIG. 2 disposed in an air medium over the microstrip lines 11 and 12. Identical components for this related art example and the previous related art example are given the same reference numerals, and explanations for the same will be omitted. Because this related art has capacitors 15 and 16 inserted at opposite ends of a section of the coupled one pair of microstrip lines 11 and 12, an odd-mode phase angle can be increased effectively, that leads the .DELTA. [=(.beta.e-.beta.o)/.beta.o] to zero, to improve the directivity.
FIG. 4 illustrates a section of a related art microstrip coupler in which a directivity is improved by using an overlay material, schematically.
Referring to FIG. 4, the another example of a related art microstrip coupler is provided with a substrate 10 with a height h and dielectric constant .di-elect cons.r, one pair of coupled microstrip lines 11 and 12, and a medium 17 with a dielectric constant .di-elect cons.r, the same as the substrate 10, overlaid to enclose the coupled one pair of microstrip lines 11 and 12. In this example of a related art, since a medium having the same dielectric constant .di-elect cons.r with the substrate is laid on the coupled microstrip lines 11 and 12, the odd-mode phase velocity is reduced, to be the same as the even-mode phase velocity, that improves the directivity. In this case, phase velocities both in the even-mode and in the odd-mode can be adjusted very accurately by adjusting a thickness and a width of the overlaid material.
However, the coupled microstrip lines in all the related arts adjusts the odd-mode phase velocity either by providing capacitors, overlaying the electric shielding metal or a medium with the same dielectric constant as that of the substrate having the coupled microstrip lines formed thereon. Therefore, though there is no problem when the microstrip lines, a passive device, is used solely, in a case when a semiconductor device, an active device, is disposed on the same substrate, there has been a problem in that it is difficult to use the microstrip lines in an MMIC(Monolithic Microwave Integrated Circuit) in which use of a shield is not generalized or in a millimeter wave integrated circuit in which use of an overlay material is limited because a thickness and kind of the overlay dielectric material affects to characteristics of the active device seriously.